The present embodiments relate to multi-dimensional transducer arrays. Beamforming may be provided for multi-dimensional capacitive membrane ultrasound transducer (CMUT) arrays.
Many clinical applications call for high volume acquisition rates. Two-dimensional arrays, especially for radiology, have enormous channel counts and element sizes that are unable to drive cables due to impedance mismatch. Cables for fully sampled two-dimensional arrays are impractically large.
A traditional array is limited in its frame rate by Nyquist spatial sampling and the scan area. When two-dimensional arrays that scan volumes are considered, the number of beams may often exceed 10,000. A typical volume may take several seconds to acquire. A data set representing a line of acoustic echoes is obtained from a transmitter firing. For a square transducer, if M beams are required to fill a plane, at least M2 beams are needed to fill a volume. A typical beam is 2 wavelengths wide, and a typical transducer may be 200 wavelengths long, giving M=100. A typical beam requires 0.2 ms to acquire. Parallel receive beamforming can help, but the data acquisition is still too slow, especially in cardiology.
A fixed transmit focus constitutes a resolution problem. In cardiology, there is no time for more than one focal zone, so the image is out of focus in most of the image. If Z focal zones are needed to improve coherence, a total of M2Z2 firings make up a volumetric image. For typical imaging depths, this results in a maximum imaging speed of 0.5/Z2 volumes per second.
Other acquisition techniques may be used. A mechanical drive may rock a one-dimensional transducer inside the probe handle. The volume may be acquired by free hand scanning a one-dimensional array, with probe position estimation performed in the imaging system or by position sensing. Beam formation may be performed in the probe handle. However, the speed of acquisition is limited by requirements of spatial beam sampling and the speed of sound in tissue. No channel information is available in the imaging system if real-time beam formation without channel data storage is performed. The only data available are the beams that are created by combining the channel data in the reconstruction algorithm. Availability of the raw channel data is important in a variety of clinical scenarios, such as phase aberration correction, motion/flow estimation, and elastography.
In a synthetic transmit aperture (STA) imager, two or more firings creates a volume data set. This can provide imaging speeds in excess of 1000 volumes per second. However, signal-to-noise ratio may suffer without advanced channel signal processing techniques.